When an electric field is externally applied to a dispersion obtained by dispersing dielectric solid particles into an electrically insulating liquid, the viscosity of the dispersion changes according to the degree of the applied voltage. This phenomenon is known as the Winslow effect or an electrorheological effect (hereinafter abbreviated as "ER effect") and is described in T. Fujita et al., Journal of Magnetism and Magnetic Materials, vol.122, pp.29-33 (North-Holland 1993). In this ER effect, the viscosity and shear stress of the dispersion as a whole apparently increase because solid particles in the dispersion are internally polarized by the action of the electric field and the polarized solid particles are statically aggregated with each other.
The fluid which produces this ER effect is called an ER fluid. Examples thereof include fluids comprising an electroinsulating liquid (e.g., paraffin oil, ester oil, ether oil, or silicon oil) and having dispersed therein (a) water-containing solid particles comprising water-absorbing or hydrophilic solid particles (e.g., cellulose, silica gel, starch, an ion-exchange resin) containing water or alcohol or (b) water-free solid particles obtained by insulating electroconductive particles (e.g., a metal, a semiconductor, or a ferroelectric substance) or electroconductive polymer particles in which polymer particles are coated with a metal.
Since the ER effect is excellent in response and controllability to applied voltage, use of the ER fluid as a working fluid for various machines and apparatus has been investigated. For example, a damper and an actuator both employing the ER fluid have been proposed.
On the other hand, a solution comprising an insulating liquid and having dispersed therein magnetic particles having a surfactant adsorbed thereon has been known as a magnetic fluid. A known representative magnetic fluid is obtained by adsorbing oleic acid onto magnetite particles and dispersing the resulting particles into kerosene.
This magnetic fluid is characterized in that the magnetic particles in the fluid attract each other by application of an external magnetic field and, as a result, the viscosity of the fluid apparently increases. Accordingly, since the viscosity of a magnetic fluid is controllable with an external magnetic field, use of a magnetic fluid as a working fluid for various machines and apparatus has been investigated in the same manner as the ER fluid described above.
A fluid having the properties of an ER fluid and those of a magnetic fluid, wherein the viscosity thereof is controllable with both an external electric field and an external magnetic field, has been reported (T. Fujita et al., Journal of Magnetism and Magnetic Materials, vol.122, pp.29-33 (North-Holland 1993)). Specifically, this reference discloses that a mixed fluid which is a mixture of a dielectric fluid containing barium titanate showing an ER effect with a kerosene-based magnetic fluid responds to both an external electric field and an external magnetic field so that the viscosity thereof can be changed.
As described above, the viscosities of an ER fluid, a magnetic fluid, and a mixture thereof can be easily controlled with an external electric field, an external magnetic field or both. Accordingly, use of these fluids as a working fluid for various machines and apparatus such as dampers and actuators has been investigated.
However, the ER fluid has the following problems. That is, the ER fluid containing water-containing solid particles has a problem that, although such ER fluid produces an ER effect at room temperatures, the ER effect is deteriorated or is hard to reveal at high temperatures because of vaporization of water. On the other hand, with regard to the ER fluid containing water-free solid particles, there is a problem that the great ER effect which is sufficient for practical use has not yet been obtained.
In the same manner, the magnetic fluid also has similar problems that a magnetic fluid having a sufficient magnetic aggregation effect has not yet been obtained.
Further, when the particles having a larger diameter are used, it is undesirable that a phase separation occurs because of the settling of the particles in the electroinsulating liquid and, as a result, the ER or magnetic effect is deteriorated or is hard to reveal.
In order to overcome the above problem, in general, two or more electroinsulating liquids are blended or an additive such as a surfactant, dispersant or antisettling agent is added in order to inhibit settling of the particles by reducing the difference in specific gravity between the particles and the dispersion medium and to control the phase separation by improving the dispersibility.
However, the technique of adjusting the difference in specific gravity between the electroinsulating liquid and the particles not only has a problem of having difficulty in specific gravity regulation, but also has a serious problem that even when an electroinsulating liquid having a large specific gravity can be prepared, this liquid is not applicable to particles having an even larger specific gravity. As a result, combinations of electroinsulating liquids with particles are limited.
The technique of improving the dispersibility of particles by adding an additive such as a surfactant, dispersant, or anti-settling agent is disadvantageous in that although such an additive is effective in improving dispersibility to some degree, the additive should be used in a considerably large amount for sufficiently homogeneously dispersing the particles having a large diameter. In the ER fluid, in particular, the addition of a large amount of such an additive may change the permittivity of the electroinsulating liquid to influence the ER effect. The addition of an additive is also undesirable because the cost increases.
On the other hand, the mixed fluid obtained by mixing an ER fluid with a magnetic fluid has both the above-described problems of the ER fluid and those of the magnetic fluid. In addition, since dielectric particles and magnetic particles coexist in the same insulating liquid, the concentration of the former particles and that of the latter particles in the fluid are low and, hence, the ER effect and the effect of magnetic aggregation in the mixed fluid are weaker than in the ER fluid alone and in the magnetic fluid alone, respectively. Accordingly, when an ER fluid is mixed with a magnetic fluid, there is a case where the viscosity characteristics of the mixed fluids are inferior to the ER fluid alone and to the magnetic fluid alone.
Even if the particle concentration is desired to be increased, the increase of the particle concentration has a limit because the concentration of all particles in a fluid is limited as described above and, hence, an increase in the concentration of either of dielectric particles and magnetic particles only results in a decrease in the concentration of the other particles. Accordingly, the effect in the mixed fluid cannot be heightened remarkably.
As described above, an ER or magnetic fluid having properties sufficient for practical use has not yet been obtained.